The experiments described in this proposal are aimed understanding the photoreceptor to On-bipolar cell synapse in the vertebrate retina. The On-bipolar cell uses a G-protein-coupled signaling mechanism to relay information it collects from the photoreceptors to the inner retina for processing. While the receptor and G-protein have been identified, the mechanisms linking these to a non-specific cation channel are unknown. I will approach the study of this cascade with the following strategy. While blocking the G-protein's action, I will determine what internal factors are necessary in On-bipolar cells to open transduction channels. To accomplish this I will make whole-cell patch clamp recordings from salamander On-bipolar cells. In particular I will test whether the native state of the transduction channel is open or closed, and whether gating mechanisms directly or indirectly involve cGMP. If cGMP is not involved, the metabolic requirements for channel opening will be identified. In parallel experiments I will determine what internal factors are necessary for maintaining the light response, or what factors are necessary for channel closure. I will also test inhibitors of other known signaling mechanisms that utilize non-specific cation channels. These two types of experiments will be corroborated by two-photon imaging of calcium in the On-bipolar dendrites, to compare the localization of "run-open" channels with those opened by the transduction mechanism. Finally, I will study how low light level information is transferred between rod photoreceptors and rod bipolar cells in the mouse retina. The signal-to-noise ratio at this synapse is maximized by a thresholding non-linearity. I will study the mechanism producing this non-linearity. First I will determine whether the mechanism is pre- or postsynaptic, and if it arises from saturation at the receptors or downstream in the transduction mechanism. Lastly, I will determine whether the non-linearity is dynamic, or whether its position can be influenced by very dim background light. [unreadable] [unreadable]